A. V. Zobnin

Dust-acoustic wave instability at the diffuse edge of radio frequency inductive low-pressure gas discharge plasma

A spontaneous excitation of a grain density wave in a dusty cloud of monodisperse particles suspended at the diffuse edge of an rf inductive gas discharge has been discovered. The main physical parameters of this wave (phase velocity, wavelength, and growth rate) and of the background plasma (distributions of the electron density, electron temperature, and space potential) were measured. A theoretical model of the observable phenomenon based on the theory of dust acoustic waves in a collisional dusty plasma correlates well with the experimental data in a broad range of experimental conditions. The influence of a varying dust grain charge on the development of the observed dusty plasma instability has been analyzed. It is shown that the necessary condition for the instability excitation is the availability of a permanent electrical field (E0 ⩾3 V/cm) in the dusty cloud region.

Determination of the ion-drag force in a complex plasma

The ion-drag force acting on dust particles in the positive column of a dc discharge is measured in the pressure range of 20-120 Pa. The force is obtained by a method which does not require a priori knowledge of the particle charge, but uses the charge gradient determined from the same experiment. The method depends only on two experimentally determined quantities: the particle drift velocity and the electric field. The comparison of experimental results with theoretical models is presented and discussed.

The project 'Plasmakristall-4' (PK-4)—a new stage in investigations of dusty plasmas under microgravity conditions: first results and future plans

The PK-4 experiment is a continuation of the successful dusty plasma experiments PK-1, PK-2 and PK-3 conducted on board of the orbital space stations Mir and International Space Station. For all these experiments it is important to avoid the strong influence of gravity, exerting an external stress on the system. Whereas PK-3 and PK-3 Plus experiments are using a planar rf capacitive discharge, PK-4 studies complex plasmas in a long cylindrical chamber with a combined dc/rf discharge. Such a configuration of the chamber will provide a particular advantage for investigation of different dynamical phenomena in complex plasmas such as sheared laminar flow of a highly nonideal dusty liquid and its transition to the turbulent regime, nozzle flow, boundary layers and instabilities, shock waves (solitons) formation and propagation, dust particle lane formation, and space dust grain separation according to their size.

Ion current on a small spherical attractive probe in a weakly ionized plasma with ion-neutral collisions (kinetic approach)

The ion flux on a small attractive sphere (the sphere radius is much less than the Debye length) and the electric potential distribution around the sphere charged in a stationary weakly ionized plasma are calculated by the direct solution of the kinetic equation with the Bhatnagar–Gross–Krook collision term. Only a weak dependence of the ion flux on the electron temperature is found. An analytical approximation of the calculated ion current–voltage characteristics is proposed for intermediate collisionalities, where neither a collisionless theory nor a continuum fluid approach is valid. This approximation was used for calculation of the floating sphere potential. A reasonable agreement of the calculated floating sphere potential with prior numerical calculations is found.

Photochemical effect in two-photon laser-induced fluorescence detection of carbon monoxide in hydrocarbon flames.

The CO formation as a result of the CO(2) photodissociation at 230.08 nm was observed by using the two-photon laser-induced fluorescence (LIF) method. The measurements were performed in a propane-air combustion product flow and in mixtures of CO(2) and O(2). The temperature dependence of the fluorescence signal caused by CO molecules, produced in the photodissociation of CO(2) molecules under the action of laser radiation at a wavelength of 230.08 nm, was measured at temperatures ranging from 1300 to 2000 K. It is shown that consideration of CO(2) photodissociation under the action of the probing radiation is necessary when one applies the two-photon LIF method for the measurement of small CO concentrations in high-temperature gas mixtures containing CO(2). As an example, a correction is given of the CO concentration profiles measured by the LIF method in the combustion product flow around a cooled metallic plate.

Grain charging in an intermediately collisional plasma

The charges of μm-size particles in the quasineutral bulk plasma of a dc discharge are determined experimentally in a pressure range between 100 and 500 Pa, spanning the transition between the weakly collisional and highly collisional (hydrodynamic) regimes, where the ion mean free path drops below the plasma screening length. The charge is determined using the force balance condition from the measured particle drift velocities in stable particle flows. A simple interpolation formula for the ion flux to the grain in the transitional regime is shown to fit quite well the experimental results.

PK-4: Complex Plasmas in Space—The Next Generation

PK-4 is an experiment designed to investigate complex plasmas in a combined dc/RF discharge under microgravity conditions on board of the International Space Station. The dc discharge is produced in a glass tube with a length of 35 cm and a diameter of 3 cm. In addition, an RF discharge can be applied by external RF coils. The setup is especially suited for studying the liquid phase of the complex plasmas, e.g., flow phenomena such as turbulence or nozzles, and forces acting on the microparticles. Experiments in the laboratory and in parabolic flights have been used to determine the charge of the microparticles as well as the ion drag force acting on them

Dust mode in collisionally dominated complex plasmas with particle drift

Experiments with flow of microparticles were conducted in a DC discharge. A sharp threshold in the neutral gas pressure for the onset of an unstable low-frequency dust wave mode was observed. Highly space- and time-resolved measurements of the microparticle flow combined with probe measurements of the plasma parameters have allowed detailed comparison with a theoretical model. The model demonstrates good qualitative and quantitative agreement with the experimental results providing accurate estimates of the particle charge.

Dust-acoustic instability in an inductive gas-discharge plasma

Spontaneous excitation of a dust-particle density wave is observed in a dust cloud levitating in the region of the diffused edge of an rf inductive low-pressure gas-discharge plasma. The main physical parameters of this wave and of the background plasma are measured. The analytic model proposed for the observed phenomenon is based on the theory of dust sound and successfully correlates with experimental data in a wide range of experimental conditions. The effect of variable charge of dust particles on the evolution of the observed dust-plasma instability is studied analytically. It is shown that the necessary condition for the development of the dust-acoustic instability is the presence of a dc electric field in the dust cloud region.

Plasmakristall-4: New complex (dusty) plasma laboratory on board the International Space Station

New complex-plasma facility, Plasmakristall-4 (PK-4), has been recently commissioned on board the International Space Station. In complex plasmas, the subsystem of μm-sized microparticles immersed in low-pressure weakly ionized gas-discharge plasmas becomes strongly coupled due to the high (103-104 e) electric charge on the microparticle surface. The microparticle subsystem of complex plasmas is available for the observation at the kinetic level, which makes complex plasmas appropriate for particle-resolved modeling of classical condensed matter phenomena. The main purpose of PK-4 is the investigation of flowing complex plasmas. To generate plasma, PK-4 makes use of a classical dc discharge in a glass tube, whose polarity can be switched with the frequency of the order of 100 Hz. This frequency is high enough not to be felt by the relatively heavy microparticles. The duty cycle of the polarity switching can be also varied allowing to vary the drift velocity of the microparticles and (when necessary) to trap them. The facility is equipped with two videocameras and illumination laser for the microparticle imaging, kaleidoscopic plasma glow observation system and minispectrometer for plasma diagnostics and various microparticle manipulation devices (e.g., powerful manipulation laser). Scientific experiments are programmed in the form of scripts written with the help of specially developed C scripting language libraries. PK-4 is mainly operated from the ground (control center CADMOS in Toulouse, France) with the support of the space station crew. Data recorded during the experiments are later on delivered to the ground on the removable hard disk drives and distributed to participating scientists for the detailed analysis.